This invention relates to the transmission and reception of digital data and more specifically to optimal decoding of digital data from phase-modulated carriers.
Digital data signals, i.e., signals consisting of "marks" and "spaces" or "ones" and "zeros" are widely used today for the efficient transmission of coded data, up to and including space transmission. Since each change of phase of the modulated carrier represents one bit, much effort has been extended to improve the accuracy of the decoded data. Probably foremost among the problems involved are noise and fading. Among methods of overcoming these problems are frequency diversity (more than one carrier frequency bearing the same data) and space diversity a single carrier frequency with multiple receiving antennas, typically spaced ten wavelengths or more apart). Many systems take advantage of the fact that the strongest signal is normally the best; some use means such as square law detection which allows the strongest signal to supply a disproportionately large part of the output signal. One disadvantage of most of the prior art is the need for frequency division. Since the carrier frequency, which was not transmitted, is required to be present for recovery of the data, the original carrier frequency must somehow be restored. Since a double carrier frequency is a resultant of most demodulation methods, the carrier frequency is normally recovered by means of a divider circuit.